There was a ruckus at lunch time. It was the best one so far. Nobody knew why they were fighting . . . You actually thought they were going to kill each other. You wanted them to stop. It wasn't funny anymore.

Newly arrived from Ghana with his mother and older sister, Harrison Opoku lives on the ninth floor of a block of flats on a London housing estate. The (second) best runner in the whole of Year 7, Harri races through his new life in his personalised trainers - the Adidas stripes drawn on in marker pen - unaware of the danger growing around him.

But when a boy is...